Laser processing apparatus suitable for formation of laser processed hole

ABSTRACT

A laser processing apparatus detects the wavelength of plasma light generated by applying a pulsed laser beam to a workpiece. A plasma detecting unit includes a first bandpass filter for passing only the wavelength of plasma light separated into a first optical path by a beam splitter, a first photodetector for detecting the light passed through the first bandpass filter, a second bandpass filter for passing only the wavelength of plasma light separated into a second optical path by the beam splitter, and a second photodetector for detecting the light passed through the second bandpass filter. In performing laser processing, the pulsed laser beam is stopped when the light intensity detected by the first photodetector is decreased and the light intensity detected by the second photodetector is increased to a peak value and then decreased to a given value slightly less than the peak value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser processing apparatus forforming a laser processed hole in a workpiece configured by bonding afirst member formed of a first material and a second member formed of asecond material, the laser processed hole extending from the firstmember to the second member.

2. Description of the Related Art

In a semiconductor device fabrication process, a plurality of crossingdivision lines called streets are formed on the front side of asubstantially disk-shaped semiconductor wafer to thereby partition aplurality of rectangular regions where devices such as ICs and LSIs arerespectively formed. The semiconductor wafer is cut along the streets tothereby divide the regions where the devices are formed from each other,thus obtaining individual semiconductor chips. For the purposes ofachieving smaller sizes and higher functionality of equipment, a modulestructure having the following configuration is in practical use. Thismodule structure is such that a plurality of devices are stacked andbonding pads provided on each device are connected to each other. Inthis module structure, through holes (via holes) are formed in asemiconductor wafer at positions corresponding to the bonding pads, anda conductive material such as aluminum is embedded in each via hole soas to be connected to the corresponding bonding pad (see Japanese PatentLaid-open No. 2003-163323, for example).

Each via hole in the semiconductor wafer mentioned above is formed byusing a drill. However, the diameter of each via hole in thesemiconductor wafer is 90 to 300 μm, so that the formation of each viahole by using a drill causes a reduction in productivity. To solve thisproblem, there has been proposed a hole forming method for a wafercomposed of a substrate and a plurality of devices formed on the frontside of the substrate, a plurality of bonding pads being formed on eachdevice, wherein a pulsed laser beam is applied to the substrate from theback side thereof to thereby efficiently form a plurality of via holesrespectively reaching the plural bonding pads (see Japanese PatentLaid-open No. 2007-67082, for example).

The wavelength of the pulsed laser beam is selected so as to have lowabsorptivity to the metal forming the bonding pads and have highabsorptivity to the material forming the substrate, such as silicon andlithium tantalate. However, in applying the pulsed laser beam to thesubstrate from the back side thereof to thereby form the via holesrespectively reaching the bonding pads, it is difficult to stop theapplication of the pulsed laser beam at the time each via hole formed inthe substrate has reached the corresponding bonding pad, causing aproblem that the bonding pads may be melted to be perforated by thepulsed laser beam. To solve this problem in the hole forming method forthe wafer disclosed in Japanese Patent Laid-open No. 2007-67082, therehas been proposed a laser processing apparatus such that a laser beam isapplied to a material to generate a plasma from the material, and aspectrum caused by this plasma and inherent in the material is detectedto thereby determine whether or not the laser beam has reached eachbonding pad formed of metal (see Japanese Patent Laid-open No.2009-125756, for example).

SUMMARY OF THE INVENTION

Each bonding pad formed of metal is located at the bottom of a fine holeformed by applying a laser beam to the substrate. Accordingly, even whenthe laser beam is applied to each bonding pad, it is difficult tocapture the moment of proper generation of the plasma from the metalforming each bonding pad and then stop the application of the laserbeam, causing a problem that each bonding pad may be melted to beperforated.

It is therefore an object of the present invention to provide a laserprocessing apparatus which can efficiently form a laser processed holein a workpiece configured by bonding a first member formed of a firstmaterial and a second member formed of a second material, the laserprocessed hole extending from the first member to the second member,without melting the second member.

In accordance with an aspect of the present invention, there is provideda laser processing apparatus for forming a laser processed hole in aworkpiece configured by bonding a first member formed of a firstmaterial and a second member formed of a second material, the laserprocessed hole extending from the first member to the second member, thelaser processing apparatus including workpiece holding means for holdingthe workpiece; laser beam applying means for applying a pulsed laserbeam to the workpiece held by the workpiece holding means; plasmadetecting means for detecting the wavelength of plasma light generatedby applying the pulsed laser beam from the laser beam applying means tothe workpiece; and control means for controlling the laser beam applyingmeans according to a detection signal from the plasma detecting means;the plasma detecting means including a beam splitter for separating theplasma light into a first optical path and a second optical path; afirst bandpass filter provided on the first optical path for passingonly the wavelength of plasma light generated from the first material; afirst photodetector for detecting the light passed through the firstbandpass filter and outputting a light intensity signal to the controlmeans; a second bandpass filter provided on the second optical path forpassing only the wavelength of plasma light generated from the secondmaterial; and a second photodetector for detecting the light passedthrough the second bandpass filter and outputting a light intensitysignal to the control means; the control means controlling the laserbeam applying means according to the light intensity signals output fromthe first and second photodetectors so that when the laser beam applyingmeans is operated to apply the pulsed laser beam to the workpiece tothereby form the laser processed hole extending from the first member tothe second member, the application of the pulsed laser beam is stoppedat the time the light intensity detected by the first photodetector isdecreased and the light intensity detected by the second photodetectoris increased to a peak value and next decreased to a given valueslightly less than the peak value.

In the laser processing apparatus according to the present invention,the plasma detecting means for detecting the wavelength of plasma lightgenerated by applying the pulsed laser beam from the laser beam applyingmeans to the workpiece includes the beam splitter for separating theplasma light into the first optical path and the second optical path,the first bandpass filter provided on the first optical path for passingonly the wavelength of plasma light generated from the first material,the first photodetector for detecting the light passed through the firstbandpass filter and outputting a light intensity signal to the controlmeans; the second bandpass filter provided on the second optical pathfor passing only the wavelength of plasma light generated from thesecond material, and the second photodetector for detecting the lightpassed through the second bandpass filter and outputting a lightintensity signal to the control means. The control means for controllingthe laser beam applying means according to a detection signal from theplasma detecting means controls the laser beam applying means accordingto the light intensity signals output from the first and secondphotodetectors so that when the laser beam applying means is operated toapply the pulsed laser beam to the workpiece to thereby form the laserprocessed hole extending from the first member to the second member, theapplication of the pulsed laser beam is stopped at the time the lightintensity detected by the first photodetector is decreased and the lightintensity detected by the second photodetector is increased to a peakvalue and next decreased to a given value slightly less than the peakvalue. Accordingly, the fine hole (laser processed hole) formed in thefirst member by the application of the pulsed laser beam can ba made toreach the second member so that the second member is completely exposedto the fine hole without being melted.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claim with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser processing apparatus accordingto a preferred embodiment of the present invention;

FIG. 2 is a block diagram showing the configuration of laser beamapplying means included in the laser processing apparatus shown in FIG.1;

FIG. 3 is a block diagram showing the configuration of plasma detectingmeans included in the laser processing apparatus shown in FIG. 1;

FIG. 4 is a block diagram showing the configuration of control meansincluded in the laser processing apparatus shown in FIG. 1;

FIG. 5 is a plan view of a wafer as a workpiece;

FIG. 6 is an enlarged plan view of an essential part of the wafer shownin FIG. 5;

FIG. 7 is a perspective view showing a condition where the wafer shownin FIG. 5 is attached to a protective tape supported to an annularframe;

FIG. 8 is a plan view showing the relation between the wafer shown inFIG. 5 and coordinates in the condition where the wafer is held at apredetermined position on a chuck table included in the laser processingapparatus shown in FIG. 1;

FIGS. 9A and 9B are partially cutaway sectional side views forillustrating a hole forming step to be performed by the laser processingapparatus shown in FIG. 1;

FIGS. 10A and 10B are views similar to FIGS. 9A and 9B, showing the stepsubsequent to the step shown in FIGS. 9A and 9B;

FIG. 11A is a graph showing an output voltage from a first photodetectorfor detecting the light intensity of plasma light generated by applyinga pulsed laser beam to a lithium tantalate substrate; and

FIG. 11B is a graph showing an output voltage from a secondphotodetector for detecting the light intensity of plasma lightgenerated by applying a pulsed laser beam to a bonding pad formed ofcopper.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the laser processing apparatus according tothe present invention will now be described in detail with reference tothe attached drawings. FIG. 1 is a perspective view of a laserprocessing apparatus 1 according to a preferred embodiment of thepresent invention. The laser processing apparatus 1 shown in FIG. 1includes a stationary base 2, a chuck table mechanism 3 for holding aworkpiece, the chuck table mechanism 3 being provided on the stationarybase 2 so as to be movable in a feeding direction (X direction) shown byan arrow X, a laser beam applying unit supporting mechanism 4 providedon the stationary base 2 so as to be movable in an indexing direction (Ydirection) shown by an arrow Y perpendicular to the X direction, and alaser beam applying unit 5 provided on the laser beam applying unitsupporting mechanism 4 so as to be movable in a focal position adjustingdirection (Z direction) shown by an arrow Z.

The chuck table mechanism 3 includes a pair of guide rails 31 providedon the stationary base 2 so as to extend parallel to each other in the Xdirection, a first slide block 32 provided on the guide rails 31 so asto be movable in the X direction, a second slide block 33 provided onthe first slide block 32 so as to be movable in the Y direction, a covertable 35 supported by a cylindrical member 34 standing on the secondslide block 33, and a chuck table 36 as workpiece holding means. Thechuck table 36 has a vacuum chuck 361 formed of a porous material. Aworkpiece such as a disk-shaped semiconductor wafer is adapted to beheld under suction on the vacuum chuck 361 by operating suction means(not shown). The chuck table 36 is rotatable by a pulse motor (notshown) provided in the cylindrical member 34. Further, the chuck table36 is provided with clamps 362 for fixing an annular frame to behereinafter described.

The lower surface of the first slide block 32 is formed with a pair ofguided grooves 321 for slidably engaging the pair of guide rails 31mentioned above. A pair of guide rails 322 are provided on the uppersurface of the first slide block 32 so as to extend parallel to eachother in the Y direction. Accordingly, the first slide block 32 ismovable in the X direction along the guide rails 31 by the slidableengagement of the guided grooves 321 with the guide rails 31. The chucktable mechanism 3 further includes feeding means 37 (X direction movingmeans) for moving the first slide block 32 in the X direction along theguide rails 31. The feeding means 37 includes an externally threaded rod371 extending parallel to the guide rails 31 so as to be interposedtherebetween and a pulse motor 372 as a drive source for rotationallydriving the externally threaded rod 371. The externally threaded rod 371is rotatably supported at one end thereof to a bearing block 373 fixedto the stationary base 2 and is connected at the other end to the outputshaft of the pulse motor 372 so as to receive the torque thereof. Theexternally threaded rod 371 is engaged with a tapped through hole formedin an internally threaded block (not shown) projecting from the lowersurface of the first slide block 32 at a central portion thereof.Accordingly, the first slide block 32 is moved in the X direction alongthe guide rails 31 by operating the pulse motor 372 to normally orreversely rotate the externally threaded rod 371.

The laser processing apparatus 1 includes X position detecting means 374for detecting the feed amount, or X position of the chuck table 36. TheX position detecting means 374 includes a linear scale 374 a extendingalong one of the guide rails 31 and a read head 374 b provided on thefirst slide block 32 and movable along the linear scale 374 a togetherwith the first slide block 32. The read head 374 b of the X positiondetecting means 374 transmits a pulse signal of one pulse every 1 μm inthis preferred embodiment to control means which will be hereinafterdescribed. This control means counts the number of pulses as the pulsesignal input from the read head 374 b to thereby detect the feed amount,or X position of the chuck table 36. In the case that the pulse motor372 is used as the drive source for the feeding means 37 as in thispreferred embodiment, the number of pulses as a drive signal output fromthe control means to the pulse motor 372 may be counted by the controlmeans to thereby detect the feed amount, or X position of the chucktable 36. In the case that a servo motor is used as the drive source forthe feeding means 37, a pulse signal output from a rotary encoder fordetecting the rotational speed of the servo motor may be sent to thecontrol means, and the number of pulses as the pulse signal input fromthe rotary encoder into the control means may be counted by the controlmeans to thereby detect the feed amount, or X position of the chucktable 36.

The lower surface of the second slide block 33 is formed with a pair ofguided grooves 331 for slidably engaging the pair of guide rails 322provided on the upper surface of the first slide block 32 as mentionedabove. Accordingly, the second slide block 33 is movable in the Ydirection along the guide rails 322 by the slidable engagement of theguided grooves 331 with the guide rails 322. The chuck table mechanism 3further includes first indexing means 38 (first Y direction movingmeans) for moving the second slide block 33 in the Y direction along theguide rails 322. The first indexing means 38 includes an externallythreaded rod 381 extending parallel to the guide rails 322 so as to beinterposed therebetween and a pulse motor 382 as a drive source forrotationally driving the externally threaded rod 381. The externallythreaded rod 381 is rotatably supported at one end thereof to a bearingblock 383 fixed to the upper surface of the first slide block 32 and isconnected at the other end to the output shaft of the pulse motor 382 soas to receive the torque thereof. The externally threaded rod 381 isengaged with a tapped through hole formed in an internally threadedblock (not shown) projecting from the lower surface of the second slideblock 33 at a central portion thereof. Accordingly, the second slideblock 33 is moved in the Y direction along the guide rails 322 byoperating the pulse motor 382 to normally or reversely rotate theexternally threaded rod 381.

The laser processing apparatus 1 includes Y position detecting means 384for detecting the index amount, or Y position of the chuck table 36. TheY position detecting means 384 includes a linear scale 384 a extendingalong one of the guide rails 322 and a read head 384 b provided on thesecond slide block 33 and movable along the linear scale 384 a togetherwith the second slide block 33. The read head 384 b of the Y positiondetecting means 384 transmits a pulse signal of one pulse every 1 μm inthis preferred embodiment to the control means. This control meanscounts the number of pulses as the pulse signal input from the read head384 b to thereby detect the index amount, or Y position of the chucktable 36. In the case that the pulse motor 382 is used as the drivesource for the first indexing means 38 as in this preferred embodiment,the number of pulses as a drive signal output from the control means tothe pulse motor 382 may be counted by the control means to therebydetect the index amount, or Y position of the chuck table 36. In thecase that a servo motor is used as the drive source for the firstindexing means 38, a pulse signal output from a rotary encoder fordetecting the rotational speed of the servo motor may be sent to thecontrol means, and the number of pulses as the pulse signal input fromthe rotary encoder into the control means may be counted by the controlmeans to thereby detect the index amount, or Y position of the chucktable 36.

The laser beam applying unit supporting mechanism 4 includes a pair ofguide rails 41 provided on the stationary base 2 so as to extendparallel to each other in the Y direction and a movable support base 42provided on the guide rails 41 so as to be movable in the Y direction.The movable support base 42 is composed of a horizontal portion 421slidably supported to the guide rails 41 and a vertical portion 422extending vertically upward from the upper surface of the horizontalportion 421. Further, a pair of guide rails 423 are provided on one sidesurface of the vertical portion 422 so as to extend parallel to eachother in the Z direction. The laser beam applying unit supportingmechanism 4 further includes second indexing means 43 (second Ydirection moving means) for moving the movable support base 42 in the Ydirection along the guide rails 41. The second indexing means 43includes an externally threaded rod 431 extending parallel to the guiderails 41 so as to be interposed therebetween and a pulse motor 432 as adrive source for rotationally driving the externally threaded rod 431.The externally threaded rod 431 is rotatably supported at one endthereof to a bearing block (not shown) fixed to the stationary base 2and is connected at the other end to the output shaft of the pulse motor432 so as to receive the torque thereof. The externally threaded rod 431is engaged with a tapped through hole formed in an internally threadedblock (not shown) projecting from the lower surface of the horizontalportion 421 at a central portion thereof. Accordingly, the movablesupport base 42 is moved in the Y direction along the guide rails 41 byoperating the pulse motor 432 to normally or reversely rotate theexternally threaded rod 431.

The laser beam applying unit 5 includes a unit holder 51 and laser beamapplying means 52 mounted to the unit holder 51. The unit holder 51 isformed with a pair of guided grooves 511 for slidably engaging the pairof guide rails 423 provided on the vertical portion 422 of the movablesupport base 42. Accordingly, the unit holder 51 is supported to themovable support base 42 so as to be movable in the Z direction by theslidable engagement of the guided grooves 511 with the guide rails 423.

The laser beam applying unit 5 further includes focal position adjustingmeans 53 (Z direction moving means) for moving the unit holder 51 alongthe guide rails 423 in the Z direction. The focal position adjustingmeans 53 includes an externally threaded rod (not shown) extendingparallel to the guide rails 423 so as to be interposed therebetween anda pulse motor 532 as a drive source for rotationally driving thisexternally threaded rod. Accordingly, the unit holder 51 and the laserbeam applying means 52 are moved in the Z direction along the guiderails 423 by operating the pulse motor 532 to normally or reverselyrotate this externally threaded rod. In this preferred embodiment, whenthe pulse motor 532 is normally operated, the laser beam applying means52 is moved upward, whereas when the pulse motor 532 is reverselyoperated, the laser beam applying means 52 is moved downward.

The laser beam applying means 52 includes a cylindrical casing 521 fixedto the unit holder 51 so as to extend in a substantially horizontaldirection, pulsed laser beam oscillating means 6 (see FIG. 2) providedin the casing 521, acoustooptic deflecting means 7 (see FIG. 2) as lightdeflecting means for deflecting the beam axis of a pulsed laser beamoscillated by the pulsed laser beam oscillating means 6 in the feedingdirection (X direction), and focusing means 8 (see FIGS. 1 and 2) forapplying the pulsed laser beam passed through the acoustoopticdeflecting means 7 to a workpiece held on the chuck table 36.

The pulsed laser beam oscillating means 6 is composed of a pulsed laseroscillator 61 such as a YAG laser oscillator or a YVO4 laser oscillatorand repetition frequency setting means 62 connected to the pulsed laseroscillator 61. The pulsed laser oscillator 61 functions to oscillate apulsed laser beam (LB) having a predetermined frequency set by therepetition frequency setting means 62. The repetition frequency settingmeans 62 functions to set the repetition frequency of the pulsed laserbeam to be oscillated by the pulsed laser oscillator 61. The pulsedlaser oscillator 61 and the repetition frequency setting means 62 of thepulsed laser beam oscillating means 6 are controlled by the controlmeans to be hereinafter described.

The acoustooptic deflecting means 7 includes an acoustooptic device 71for deflecting the beam axis of the pulsed laser beam (LB) oscillated bythe pulsed laser beam oscillating means 6, an RF oscillator 72 forgenerating an RF (radio frequency) signal to be applied to theacoustooptic device 71, an RF amplifier 73 for amplifying the power ofthe RF signal generated by the RF oscillator 72 and applying theamplified RF signal to the acoustooptic device 71, deflection angleadjusting means 74 for adjusting the frequency of the RF signal to begenerated by the RF oscillator 72, and power adjusting means 75 foradjusting the amplitude of the RF signal to be generated by the RFoscillator 72. The acoustooptic device 71 can adjust the angle ofdeflection of the pulsed laser beam according to the frequency of the RFsignal applied and can also adjust the power of the pulsed laser beamaccording to the amplitude of the RF signal applied. The acoustoopticdeflecting means 7 as the light deflecting means may be replaced byelectrooptic deflecting means using an electrooptic device. Thedeflection angle adjusting means 74 and the power adjusting means 75 arecontrolled by the control means to be described later.

The laser beam applying merans 52 further includes laser beam absorbingmeans 76 for absorbing the pulsed laser beam deflected by theacoustooptic device 71 as shown by a broken line in FIG. 2 in the casethat an RF signal having a predetermined frequency is applied to theacoustooptic device 71.

The focusing means 8 is mounted at the front end of the casing 521 andit includes a direction changing mirror 81 for downwardly changing thetraveling direction of the pulsed laser beam deflected by theacoustooptic deflecting means 7 and a focusing lens 82 provided by atelecentric lens for focusing the pulsed laser beam whose travelingdirection has been changed by the direction changing mirror 81.

The operation of the laser beam applying means 52 will now be describedwith reference to FIG. 2. In the case that a voltage of 5 V, forexample, is applied from the control means to the deflection angleadjusting means 74 of the acoustooptic deflecting means 7 and an RFsignal having a frequency corresponding to 5 V is applied to theacoustooptic device 71, the pulsed laser beam oscillated by the pulsedlaser beam oscillating means 6 is deflected in beam axis as shown by asingle dot & dash line in FIG. 2 and focused at a focal point Pa. In thecase that a voltage of 10 V, for example, is applied from the controlmeans to the deflection angle adjusting means 74 and an RF signal havinga frequency corresponding to 10 V is applied to the acoustooptic device71, the pulsed laser beam oscillated by the pulsed laser beamoscillating means 6 is deflected in beam axis as shown by a solid linein FIG. 2 and focused at a focal point Pb displaced from the focal pointPa to the left as viewed in FIG. 2 in the X direction by a predeterminedamount. In the case that a voltage of 15 V, for example, is applied fromthe control means to the deflection angle adjusting means 74 and an RFsignal having a frequency corresponding to 15 V is applied to theacoustooptic device 71, the pulsed laser beam oscillated by the pulsedlaser beam oscillating means 6 is deflected in beam axis as shown by adouble dot & dash line in FIG. 2 and focused at a focal point Pcdisplaced from the focal point Pb to the left as viewed in FIG. 2 in theX direction by a predetermined amount. Further, in the case that avoltage of 0 V, for example, is applied from the control means to thedeflection angle adjusting means 74 of the acoustooptic deflecting means7 and an RF signal having a frequency corresponding to 0 V is applied tothe acoustooptic device 71, the pulsed laser beam oscillated by thepulsed laser beam oscillating means 6 is led to the laser beam absorbingmeans 76 as shown by a broken line in FIG. 2. Thus, the pulsed laserbeam oscillated by the pulsed laser beam oscillating means 6 isdeflected to different positions in the X direction by the acoustoopticdevice 71 according to the voltage applied to the deflection angleadjusting means 74.

Referring back to FIG. 1, the laser processing apparatus 1 furtherincludes plasma detecting means 9 mounted on the casing 521 of the laserbeam applying means 52 constituting the laser beam applying unit 5 fordetecting plasma light generated by applying the laser beam from thelaser beam applying means 52 to the workpiece. As shown in FIG. 3, theplasma detecting means 9 includes plasma capturing means 91 forcapturing plasma light generated by applying the laser beam from thefocusing means 8 of the laser beam applying means 52 to the workpiece Wheld on the chuck table 36, a beam splitter 92 for separating the plasmalight captured by the plasma capturing means 91 into a first opticalpath 92 a and a second optical path 92 b, a first bandpass filter 93provided on the first optical path 92 a for passing only the lighthaving a first set wavelength (the wavelength to be generated from afirst material forming a first member of the workpiece to be hereinafterdescribed), a first photodetector 94 for detecting the light passedthrough the first bandpass filter 93 to output a light intensity signal,a direction changing mirror 95 provided on the second optical path 92 b,a second bandpass filter 96 for passing only the light having a secondset wavelength (the wavelength to be generated from a second materialforming a second member of the workpiece to be hereinafter described)after the light (plasma light) being reflected on the direction changingmirror 95, and a second photodetector 97 for detecting the light passedthrough the second bandpass filter 96 to output a light intensitysignal.

The plasma capturing means 91 is composed of a focusing lens 911 and alens case 912 for accommodating the focusing lens 911. As shown in FIG.1, the lens case 912 is mounted on the casing 521 of the laser beamapplying means 52. Further, as shown in FIG. 1, the lens case 912 isprovided with an angle adjusting knob 913 for adjusting the installationangle of the focusing lens 911. The first bandpass filter 93 is soconfigured as to pass the light having a wavelength range of 660 to 680nm because only the wavelength (670 nm) of plasma light to be generatedfrom lithium tantalate is to be passed. On the other hand, the secondbandpass filter 96 is so configured as to pass the light having awavelength range of 500 to 540 nm because only the wavelength (515 nm)of plasma light to be generated from copper is to be passed. The firstphotodetector 94 detects the light passed through the first bandpassfilter 93 and outputs to the control means a voltage signalcorresponding to the intensity of the light detected. Similarly, thesecond photodetector 97 detects the light passed through the secondbandpass filter 96 and outputs to the control means a voltage signalcorresponding to the intensity of the light detected.

Referring back to FIG. 1, the laser processing apparatus 1 furtherincludes imaging means 11 provided at the front end portion of thecasing 521 for imaging a subject area of the workpiece to belaser-processed by the laser beam applying means 52. The imaging means11 includes an ordinary imaging device (CCD) for imaging the workpieceby using visible light, infrared light applying means for applyinginfrared light to the workpiece, an optical system for capturing theinfrared light applied to the workpiece by the infrared light applyingmeans, and an imaging device (infrared CCD) for outputting an electricalsignal corresponding to the infrared light captured by the opticalsystem. An image signal output from the imaging means 11 is transmittedto the control means 20 (see FIG. 4).

The laser processing apparatus 1 includes the control means 20 shown inFIG. 4. The control means 20 is configured by a computer, and itincludes a central processing unit (CPU) 201 for performing operationalprocessing according to a control program, a read only memory (ROM) 202preliminarily storing the control program, a random access memory (RAM)203 for storing a control map, data on design value for the workpiece,the results of computation, etc., a counter 204, an input interface 205,and an output interface 206. Detection signals from the X positiondetecting means 374, the Y position detecting means 384, the first andsecond photodetectors 94 and 97 of the plasma detecting means 9, and theimaging means 11 are input into the input interface 205 of the controlmeans 20. On the other hand, control signals are output from the outputinterface 206 of the control means 20 to the pulse motor 372, the pulsemotor 382, the pulse motor 432, the pulse motor 532, the pulsed laseroscillator 61 and the repetition frequency setting means 62 of thepulsed laser beam oscillating means 6 constituting the laser beamapplying means 52, and the deflection angle adjusting means 74 and thepower adjusting means 75 of the acoustooptic deflecting means 7constituting the laser beam applying means 52. The random access memory(RAM) 203 includes a first memory area 203 a for storing the relationbetween the material of the workpiece and the wavelength of plasmalight, a second memory area 203 b for storing data on design value for awafer to be hereinafter described, and other memory areas.

The operation of the laser processing apparatus 1 configured above willnow be described. FIG. 5 is a plan view of a wafer 30 as the workpieceto be laser-processed, showing the front side of the wafer 30. The wafer30 is formed from a lithium tantalate substrate 300 (first member)having a thickness of 300 μm, for example. A plurality of crossingdivision lines 301 are formed on the front side 300 a of the substrate300, thereby partitioning a plurality of rectangular regions where aplurality of devices 302 are respectively formed. All of the devices 302have the same configuration. As shown in FIG. 6, a plurality of bondingpads 303 (303 a to 303 j) (second member) are formed on the front sideof each device 302. In this preferred embodiment, these bonding pads 303(303 a to 303 j) are formed of copper. The bonding pads 303 a and 303 fhave the same X position, the bonding pads 303 b and 303 g have the sameX position, the bonding pads 303 c and 303 h have the same X position,the bonding pads 303 d and 303 i have the same X position, and thebonding pads 303 e and 303 j have the same X position. The wafer 30 isprocessed by the laser beam to form a laser processed hole (via hole)extending from the back side 300 b of the substrate 300 to each bondingpad 303 (303 a to 303 j).

In each device 302, the bonding pads 303 (303 a to 303 j) are equallyspaced at given intervals A in the X direction (horizontal direction asviewed in FIG. 6). More specifically, the spacing A between the bondingpads 303 a and 303 b is equal to the spacing between the bonding pads303 b and 303 c, the spacing between the bonding pads 303 c and 303 d,the spacing between the bonding pads 303 d and 303 e, the spacingbetween the bonding pads 303 f and 303 g, the spacing between thebonding pads 303 g and 303 h, the spacing between the bonding pads 303 hand 303 i, and the spacing between the bonding pads 303 i and 303 j.Further, in the adjacent devices 302 opposed in the X direction withrespect to the vertical division line 301, the adjacent bonding pads 303are equally spaced at given intervals B in the X direction. Morespecifically, the spacing B between the bonding pads 303 e and 303 a inthe adjacent devices 302 in the X direction is equal to the spacingbetween the bonding pads 303 j and 303 f in the adjacent devices 302 inthe X direction.

Further, in each device 302, the bonding pads 303 (303 a to 303 j) areequally spaced at given intervals C in the Y direction (verticaldirection as viewed in FIG. 6). More specifically, the spacing C betweenthe bonding pads 303 a and 303 f is equal to the spacing between thebonding pads 303 b and 303 g, the spacing between the bonding pads 303 cand 303 h, the spacing between the bonding pads 303 d and 303 i, and thespacing between the bonding pads 303 e and 303 j. Further, in theadjacent devices 302 opposed in the Y direction with respect to thehorizontal division line 301, the adjacent bonding pads 303 are equallyspaced at given intervals D in the Y direction. More specifically, thespacing D between the bonding pads 303 f and 303 a in the adjacentdevices 302 in the Y direction is equal to the spacing between thebonding pads 303 g and 303 b in the adjacent devices 302 in the Ydirection, the spacing between the bonding pads 303 h and 303 c in theadjacent devices 302 in the Y direction, the spacing between the bondingpads 303 i and 303 d in the adjacent devices 302 in the Y direction, andthe spacing between the bonding pads 303 j and 303 e in the adjacentdevices 302 in the Y direction. Referring to FIG. 5, symbols E1 to Endenote the rows of the devices 302 formed on the front side of the wafer30, and symbols Fl to Fn denote the columns of the devices 302, where nis the integer greater than 1. The number of devices 302 in each of therows E1 to En and the columns Fl to Fn, the values of the spacings A, B,C, and D mentioned above, and the X and Y coordinate values for all thedevices 302 are stored in the second memory area 203 b of the randomaccess memory (RAM) 203.

There will now be described a laser processing operation of processingthe wafer 30 by using the laser processing apparatus 1 to form a laserprocessed hole (via hole) extending from the back side 300 b of thesubstrate 300 to each of the bonding pads 303 (303 a to 303 j) in eachdevice 302. As shown in FIG. 7, the wafer 30 is supported through aprotective tape 50 to an annular frame 40 in such a manner that thefront side 300 a of the substrate 300 constituting the wafer 30 isattached to the protective tape 50. The protective tape 50 ispreliminarily supported at its outer circumferential portion to theannular frame 40. The protective tape 50 is formed from a syntheticresin sheet such as a polyolefin sheet. Accordingly, the back side 300 bof the substrate 300 constituting the wafer 30 attached to theprotective tape 50 is oriented upward. The wafer 30 supported throughthe protective tape 50 to the annular frame 40 is placed on the chucktable 36 of the laser processing apparatus 1 shown in FIG. 1 in thecondition where the protective tape 50 comes into contact with the uppersurface of the chuck table 36. Thereafter, the suction means not shownis operated to hold the wafer 30 through the protective tape 50 on thechuck table 36 under suction. Accordingly, the wafer 30 is held on thechuck table 36 in the condition where the back side 300 b of thesubstrate 300 constituting the wafer 30 is oriented upward. Further, theannular frame 40 is fixed by the clamps 362.

Thereafter, the feeding means 37 is operated to move the chuck table 36holding the wafer 30 to a position directly below the imaging means 11.In the condition where the chuck table 36 is positioned directly belowthe imaging means 11, the wafer 30 is set at the coordinate positionshown in FIG. 8. In this condition, an alignment operation is performedto detect whether or not the crossing division lines 301 of the wafer 30held on the chuck table 36 are parallel to the X direction and the Ydirection. That is, the imaging means 11 is operated to image the wafer30 held on the chuck table 36 and perform image processing such aspattern matching, thus performing the alignment operation. Although thefront side 300 a on which the division lines 301 of the wafer 30 areformed is oriented downward, the division lines 301 can be imaged fromthe back side 300 b through the substrate 300 of the wafer 30 becausethe lithium tantalate substrate 300 constituting the wafer 30 istransparent.

Thereafter, the chuck table 36 is moved to position the leftmost device302 on the uppermost row E1 as viewed in FIG. 8 directly below theimaging means 11. Further, the left upper bonding pad 303 a of thebonding pads 303 (303 a to 303 j) in this leftmost device 302 as viewedin FIG. 8 is positioned directly below the imaging means 11. In thiscondition, the bonding pad 303 a is detected by the imaging means 11 andthe coordinate value (a1) for the bonding pad 303 a is sent as a firstfeed start position coordinate value to the control means 20. Thecontrol means 20 stores this coordinate value (a1) as the first feedstart position coordinate value into the random access memory (RAM) 203(feed start position detecting step). The imaging means 11 and thefocusing means 8 of the laser beam applying means 52 are spaced apredetermined distance in the X direction. Accordingly, the sum of the Xcoordinate value constituting the first feed start position coordinatevalue and the above predetermined distance between the imaging means 11and the focusing means 8 is stored into the RAM 203.

After detecting the first feed start position coordinate value (a1) inthe leftmost device 302 on the uppermost row E1 as viewed in FIG. 8, thechuck table 36 is moved in the Y direction by the pitch of the divisionlines 301 and also moved in the X direction to position the leftmostdevice 302 on the second uppermost row E2 as viewed in FIG. 8 directlybelow the imaging means 11. Further, the left upper bonding pad 303 a ofthe bonding pads 303 (303 a to 303 j) in this leftmost device 302 asviewed in FIG. 8 is positioned directly below the imaging means 11. Inthis condition, the bonding pad 303 a is detected by the imaging means11 and the coordinate value (a2) for the bonding pad 303 a is sent as asecond feed start position coordinate value to the control means 20. Thecontrol means 20 stores this coordinate value (a2) as the second feedstart position coordinate value into the random access memory (RAM) 203.As mentioned above, the imaging means 11 and the focusing means 8 arespaced a predetermined distance in the X direction. Accordingly, the sumof the X coordinate value constituting the second feed start positioncoordinate value and the above distance between the imaging means 11 andthe focusing means 8 is stored into the RAM 203. Thereafter, the controlmeans 20 repeatedly performs the indexing operation (stepwise movementin the Y direction) and the feed start position detecting step mentionedabove until the lowermost row En as viewed in FIG. 8 to detect the feedstart position coordinate values (a3 to an) for the leftmost devices 302on the other rows (E3 to En) and store these coordinate values into therandom access memory (RAM) 203. Of all the devices 302 formed on thewafer 30, the leftmost device 302 on the lowermost row En as viewed inFIG. 8 is set as a measurement device, and the feed start positioncoordinate value (an) for this measurement device 302 is stored as ameasurement position coordinate value (an) into the random access memory(RAM) 203.

After performing the feed start position detecting step mentioned above,a hole forming step is performed to form a laser processed hole (viahole) through the substrate 300 of the wafer 30 at each of the bondingpads 303 (303 a to 303 j) formed in each device 302. In this holeforming step, the feeding means 37 is first operated to move the chucktable 36 so that the bonding pad 303 a corresponding to the first feedstart position coordinate value (a1) stored in the random access memory(RAM) 203 is positioned directly below the focusing means 8 of the laserbeam applying means 52. FIG. 9A shows this condition where the bondingpad 303 a corresponding to the first feed start position coordinatevalue (a1) is positioned directly below the focusing means 8. Then, thefeeding means 37 is controlled by the control means 20 to feed the chucktable 36 at a predetermined feed speed in the direction shown by anarrow X1 in FIG. 9A. At the same time, the laser beam applying means 52is controlled by the control means 20 to apply a pulsed laser beam fromthe focusing means 8 to the wafer 30. The focal point P of the pulsedlaser beam to be applied from the focusing means 8 is set near the backside 300 b (upper surface as viewed in FIG. 9A) of the substrate 300 ofthe wafer 30. At this time, the control means 20 outputs a controlsignal for controlling the deflection angle adjusting means 74 and thepower adjusting means 75 of the acoustooptic deflecting means 7according to a detection signal from the read head 374 b of the Xposition detecting means 374.

On the other hand, the RF oscillator 72 outputs an RF signalcorresponding to the control signal from the deflection angle adjustingmeans 74 and the power adjusting means 75. The power of the RF signaloutput from the RF oscillator 72 is amplified by the RF amplifier 73,and the amplified RF signal is applied to the acoustooptic device 71. Asa result, the acoustooptic device 71 deflects the beam axis of thepulsed laser beam oscillated by the pulsed laser beam oscillating means6 in the range from the position shown by the single dot & dash line inFIG. 2 to the position shown by the double dot & dash line in FIG. 2 insynchronism with the feed speed of the chuck table 36. As a result, thepulsed laser beam having a predetermined power can be applied to thewafer 30 at the position of the bonding pad 303 a corresponding to thefirst feed start position coordinate value (a1).

For example, the hole forming step mentioned above may be performedunder the following processing conditions.

Light source: LD pumped Q-switched Nd:YVO4 pulsed laser

Wavelength: 532 nm

Average power: 2 W

Repetition frequency: 50 kHz

Pulse width: 10 ps

Focused spot diameter: 15 μm

In performing the hole forming step, the control means 20 operates thecounter 204 to count the number of shots of the pulsed laser beamoscillated by the pulsed laser beam oscillating means 6 and alsooperates the plasma detecting means 9 to input a light intensity signalfrom the first photodetector 94. The light intensity signal to be outputfrom the first photodetector 94 will now be described. When the pulsedlaser beam is applied to the lithium tantalate substrate 300constituting the wafer 30, plasma light having a wavelength of 670 nm isgenerated from the substrate 300. This plasma light having a wavelengthof 670 nm is focused by the focusing lens 911 of the plasma capturingmeans 91 constituting the plasma detecting means 9 and then passedthrough the first bandpass filter 93 to reach the first photodetector94.

FIG. 11A shows an output voltage from the first photodetector 94 fordetecting the light intensity of plasma light generated by applying thepulsed laser beam to the lithium tantalate substrate 300. In FIG. 11A,the horizontal axis represents the number of shots of the pulsed laserbeam, and the vertical axis represents voltage (V). In the preferredembodiment shown in FIG. 11A, the output voltage from the firstphotodetector 94 is substantially constant at about 2.5 V until thenumber of shots of the pulsed laser beam becomes about 80 to 85. Whenthe number of shots of the pulsed laser beam exceeds 85 to approach theend of the hole forming step, the output voltage from the firstphotodetector 94 is rapidly decreased. When the number of shots of thepulsed laser beam reaches 130, the output voltage from the firstphotodetector 94 becomes zero, which means that a through hole has beenformed in the lithium tantalate substrate 300 and the pulsed laser beamhas reached the bonding pad 303 a.

FIG. 11B shows an output voltage from the second photodetector 97 fordetecting the light intensity of plasma light generated by applying thepulsed laser beam to the bonding pad 303 a formed of copper. In FIG.11B, the horizontal axis represents the number of shots of the pulsedlaser beam, and the vertical axis represents voltage (V). In thepreferred embodiment shown in FIG. 11B, the output voltage from thesecond photodetector 97 increases from the time the number of shots ofthe pulsed laser beam has become 80 to 85. When the number of shots ofthe pulsed laser beam becomes 115, the output voltage from the secondphotodetector 97 reaches a peak value (1.1 V). Thereafter, the outputvoltage from the second photodetector 97 is decreased. When the numberof shots of the pulsed laser beam reaches 140, the output voltage fromthe second photodetector 97 becomes zero, which means that a throughhole has been formed in the bonding pad 303 a. Accordingly, theapplication of the pulsed laser beam is stopped at the time the outputvoltage from the first photodetector 94 is decreased and the outputvoltage from the second photodetector 97 is increased to a peak value(e.g., 1.1 V) and next decreased to a given value (e.g., 1.09 to 1.08 V)slightly less than the peak value. As a result, a through hole is formedin the lithium tantalate substrate 300 by the pulsed laser beam, and thebonding pad 303 a can be exposed to the through hole without beingmelted to be perforated.

The control means 20 inputs a detection signal from the read head 374 bof the X position detecting means 374 and counts this detection signalthrough the counter 204. When the count value by the counter 204 reachesthe coordinate value for the next bonding pad 303 b in the X direction,the control means 20 controls the laser beam applying means 52 tosimilarly perform the hole forming step. Thereafter, every time thecount value by the counter 204 reaches the coordinate value for eachbonding pad 303 (303 c to 303 e), the control means 20 operates thelaser beam applying means 52 to similarly perform the hole forming step.When the hole forming step is performed at the position of the rightmostbonding pad 303 e in the rightmost device 302 on the uppermost row E1 asshown in FIG. 9B, the operation of the feeding means 37 is stopped tostop the movement of the chuck table 36. As a result, a plurality oflaser processed holes 304 respectively reaching the bonding pads 303 ato 303 e in each device 302 on the uppermost row E1 are formed throughthe lithium tantalate substrate 300 of the wafer 30 as shown in FIG. 9B.

Thereafter, the control means 20 controls the first indexing means 38 toindex the focusing means 8 of the laser beam applying means 52 in thedirection perpendicular to the sheet plane of FIG. 9B, i.e., in the Ydirection. On the other hand, the control means 20 inputs a detectionsignal from the read head 384 b of the Y position detecting means 384and counts this detection signal through the counter 204. When the countvalue by the counter 204 reaches a value corresponding to the spacing Cof the bonding pads 303 in the Y direction shown in FIG. 6, theoperation of the first indexing means 38 is stopped to stop the indexingof the focusing means 8. As a result, the focusing means 8 is positioneddirectly above the bonding pad 303 j (see FIG. 6) opposed to the bondingpad 303 e in the Y direction. FIG. 10A shows this condition where thefocusing means 8 is positioned directly above the bonding pad 303 j inthe rightmost device 302 on the uppermost row E1.

Thereafter, the control means 20 controls the feeding means 37 to feedthe chuck table 36 in the direction shown by an arrow X2 in FIG. 10A ata predetermined feed speed. At the same time, the control means 20operates the laser beam applying means 52 to perform the hole formingstep. As described above, the control means 20 inputs a detection signalfrom the read head 374 b of the X position detecting means 374 andcounts this detection signal through the counter 204. Every time thecount value by the counter 204 reaches the coordinate value for eachbonding pad 303 (303 j to 303 f), the control means 20 operates thelaser beam applying means 52 to similarly perform the hole forming step.When the hole forming step is performed at the position of the leftmostbonding pad 303 f in the leftmost device 302 on the uppermost row E1 asshown in FIG. 10B, the operation of the feeding means 37 is stopped tostop the movement of the chuck table 36. As a result, a plurality oflaser processed holes 304 respectively reaching the bonding pads 303 jto 303 f in each device 302 on the uppermost row E1 are formed throughthe lithium tantalate substrate 300 of the wafer 30 as shown in FIG.10B.

Thus, the laser processed holes 304 are formed through the substrate 300of the wafer 30 at the positions corresponding to the bonding pads 303in each device 302 on the uppermost row E1 as described above.Thereafter, the control means 20 operates the feeding means 37 and thefirst indexing means 38 to position the bonding pad 303 a correspondingto the second feed start position coordinate value (a2) directly belowthe focusing means 8 of the laser beam applying means 52, wherein thebonding pad 303 a corresponding to the second feed start positioncoordinate value (a2) is formed in the leftmost device 302 on the seconduppermost row E2 and the second feed start position coordinate value(a2) is stored in the random access memory (RAM) 203. Thereafter, thecontrol means 20 controls the laser beam applying means 52, the feedingmeans 37, and the first indexing means 38 to perform the hole formingstep at the positions corresponding to the bonding pads 303 in the otherdevices 302 on the second uppermost row E2. Thereafter, the hole formingstep is similarly performed at the positions corresponding to thebonding pads 303 in all the devices 302 on the other rows E3 to En. As aresult, a plurality of laser processed holes 304 respectively reachingthe bonding pads 303 in all the devices 302 on the other rows E3 to Enare formed through the lithium tantalate substrate 300 of the wafer 30.

In the hole forming step mentioned above, the pulsed laser beam is notapplied to the areas of the wafer 30 corresponding to the spacing A andthe spacing B in the X direction shown in FIG. 6 and the areas of thewafer 30 corresponding to the spacing C and the spacing D in the Ydirection shown in FIG. 6. To avoid the application of the pulsed laserbeam to these areas corresponding to the spacings A, B, C, and D in thewafer 30, the control means 20 applies a voltage of 0 V to thedeflection angle adjusting means 74 of the acoustooptic deflecting means7. As a result, an RF signal having a frequency corresponding to 0 V isapplied to the acoustooptic device 71, so that the pulsed laser beam(LB) oscillated by the pulsed laser beam oscillating means 6 is led tothe laser beam absorbing means 76 as shown by the broken line in FIG. 2,thereby avoiding the application of the pulsed laser beam to the wafer30.

While a specific preferred embodiment of the present invention has beendescribed, it should be noted that the present invention is not limitedto the above preferred embodiment, but various modifications may be madewithin the scope of the present invention. For example, in the abovepreferred embodiment, the plural laser processed holes are formed in thewafer including the substrate (first member), the plural devices formedon the front side of the substrate (first member), and the pluralbonding pads (second member) provided on each device, wherein the plurallaser processed holes respectively extend from the back side of thesubstrate (first member) to the plural bonding pads (second member).However, the present invention is widely applicable to the case offorming a laser processed hole in a workpiece configured by bonding afirst member formed of a first material and a second member formed of asecond material, wherein the laser processed hole extends from the firstmember to the second member.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claim and all changes and modifications as fall within theequivalence of the scope of the claim are therefore to be embraced bythe invention.

What is claimed is:
 1. A laser processing apparatus for forming a laserprocessed hole in a workpiece configured by bonding a first memberformed of a first material and a second member formed of a secondmaterial, the laser processed hole extending from the first member tothe second member, the laser processing apparatus comprising: workpieceholding means for holding the workpiece; laser beam applying means forapplying a pulsed laser beam to the workpiece held by the workpieceholding means; plasma detecting means for detecting the wavelength ofplasma light generated by applying the pulsed laser beam from the laserbeam applying means to the workpiece; and control means for controllingthe laser beam applying means according to a detection signal from theplasma detecting means; the plasma detecting means including a beamsplitter for separating the plasma light into a first optical path and asecond optical path, a first bandpass filter provided on the firstoptical path for passing only the wavelength of plasma light generatedfrom the first material, a first photodetector for detecting the lightpassed through the first bandpass filter and outputting a lightintensity signal to the control means, a second bandpass filter providedon the second optical path for passing only the wavelength of plasmalight generated from the second material, and a second photodetector fordetecting the light passed through the second bandpass filter andoutputting a light intensity signal to the control means; and thecontrol means controlling the laser beam applying means according to thelight intensity signals output from the first and second photodetectorsso that when the laser beam applying means is operated to apply thepulsed laser beam to the workpiece to thereby form the laser processedhole extending from the first member to the second member, theapplication of the pulsed laser beam is stopped at the time the lightintensity detected by the first photodetector is decreased and the lightintensity detected by the second photodetector is increased to a peakvalue and next decreased to a given value slightly less than the peakvalue.